Use the MouseDown event of the UltraFormattedTextEditor, with code such as this:

private void txtFormatted_MouseDown(object sender, MouseEventArgs e)
{
    if (!(txtFormatted.UIElement.ElementFromPoint(e.Location) is ImageUIElement))
        return;

    if (e.Button == System.Windows.Forms.MouseButtons.Left)
    {
        MessageBox.Show("Left mouse down over image.");
    }
    else if (e.Button == System.Windows.Forms.MouseButtons.Right)
    {
        MessageBox.Show("Right mouse down over image.");
    }
}